Research On Modular Multilevel Converter For VSC-HVDC System

With the development of power electronics, SIEMENS puts forward a topology of Modular Multilevel Converter (MMC) with multiple cascaded sub-modules in the field of High Voltage Direct Current (VSC-HVDC) transmission. Its modular structure can not only ease of integration and installation, but also let power devices work at lower voltage levels, thus greatly improving the installed capacity and voltage rating. MMC is equipped with DC bus which is especially appropriate for VSC-HVDC and back to back converter. Due to the fact that the voltage unbalance of sub-modules and circulating current problems has seriously affected the control accuracy and dynamic performance of MMC, current studies on MMC controlling strategies mainly concentrate on the voltage balance of sub-modules or circulating-current restraining. However, the realization of circulating-current restraining and voltage balance of sub-modules at the same time still remains a problem. Moreover, few researchers have touched upon the improvement of switch frequency inconsistency caused by difference of sub-module parameters under nearest level approximation modeling.This paper begins with the working principle of MMC, proposing the equivalent voltage source circuit and its four-quadrant operation mechanism, analyzes reasons for circulating current generation as well as the voltage balance of sub-modules. Based on the requirements for device design of low voltage physical modeling experiment, it provides selecting and calculating schemes on DC bus voltage, the leg inductance and sub-module capacitor; Based on traditional frequency controlling strategies on side voltage current feedbacks, it studies the working mechanism of MMC and presents an arm current direct control scheme of MMC on the basis of multi-hierarchy approximation scheme for voltage control. This scheme offers respective current parameters for each arm, and realizes MMC circulating current control by directly controlling a zero-circulating current arm whose upper and lower arm have opposite extremum; the outer voltage loop is under hierarchical voltage control. It adopts the nearest level approximation mode for21-level MMC typology. The recent study builds a small-signal transfer function model of MMC, and provides design method and empirical tuning formula for regulators of feedback control scheme and capacitor voltage balance control strategy. It provides an optimization plan on the inconsistency of switch frequency caused by sub-module parameter differences after calculating the triggering frequency of controller and modulation parameters.Grid-connected MMC inverter as an example, with the capacitor voltage balance control strategy, simulation and verification on arm current direct control scheme based on nearest level approximation are conducted. The experiment results verified the proposed control strategy. Simulation results show that the proposed arm current direct control scheme of MMC has excellent stead-state and dynamic characteristics. It effectively restraints internal loop three-phase circulating current as well as voltage unbalance of sub-modules, reduces voltage wave of sub-modules and harmonic content of alternating current, and finally verifies the feasibility and effectiveness of the proposed scheme.